1. Field of the Invention
The present invention relates to a tripod type constant velocity universal joint to be incorporated in for example a driving system of an automobile, for transmission of a rotational force mainly between non-linearly oriented rotating shafts.
2. Description of the Related Art
FIG. 8 is a cross-sectional view showing a conventional tripod type constant velocity universal joint 41, for example disclosed in JP-A No. 2002-147482, including an inner joint member 44 having three trunnions 45 radially projecting from its outer circumferential surface, respectively engaged via a roller assembly 46 in each of three track grooves 43 provided on an inner circumferential surface of an outer joint member 42, so as to transmit a torque between the outer joint member 42 and the inner joint member 44, while allowing angular and axial displacement with respect to each other.
The tripod type constant velocity universal joint 41 is of a double roller type, in which the roller assembly 46 includes two rollers, namely an inner roller 47 and an outer roller 48. The inner roller 47 is spherically and pivotally fitted to the trunnion 45. The outer roller 48 is relatively rotatable and movable in an axial direction with respect to the inner roller 47, via a plurality of needle rollers 49 interposed between the cylindrical outer circumferential surface of the inner roller 47 and the cylindrical inner circumferential surface of the outer roller 48. On the inner circumferential surface of the outer joint member 42, a roller guide section 50 is provided adjacent to each circumferential edge of the track groove 43. The roller guide section 50 is a curved recess having an arc-shaped cross-section. The outer circumferential surface of the outer roller 48 has a generatrix curvature radius generally the same as that of the roller guide section 50, so that the surfaces of the outer roller 48 and the roller guide section 50 are closely butted when a torque Is applied.
Referring to FIG. 9, when a torque is applied to the tripod type constant velocity universal joint 41 at an operating angle θ, i.e. when an axial line of the outer joint member 42 and an axial line of the inner joint member 44 are inclined by the angle θ, the trunnions 45 swing along the respective mating track grooves 43 as indicated by the arrow a in FIG. 9, along with the rotation of the inner joint member 44. At this stage, the outer roller 48 reciprocates along the track groove 43, while rolling on the load side roller guide section 50.
When the trunnion 45 swings along the track groove 43, the inner roller 47 pivotally rotates with respect to the trunnion 45, thereby generating a frictional force against the trunnion 45. The frictional force in turn generates a spin moment M1 in the roller assembly 46, so as to change an inclination φ1 (ωt) of the outer joint member 42 in an axial cross-section.
The swinging motion of the trunnion 45 along the track groove 43 also displaces a position of the front end portion of the trunnion 45 in a radial direction with respect to the outer joint member 42. At this moment, the inner roller 47 follows the trunnion 45 so as to be displaced in a radial direction of the outer joint member 42, while the outer roller 48 is detained by the track groove 43 and thus inhibited from moving in a radial direction of the outer joint member 42, and hence the inner roller 47 and the outer roller 48 relatively move in an axial direction. Therefore, as shown in FIG. 10, a line of action of a force F2 loaded on the inner roller 47 from the trunnion 45 is offset with respect to a line of action of a force F1 loaded on the outer roller 48 from the roller guide section 50, which generates a spin moment M2 in the roller assembly 46 so as to change an inclination φ2 (ωt) of the outer joint member 42 in a cross-section orthogonal to an axial line.
Normally the spin moments M1, M2 are simultaneously generated, and the inclinations φ1 (ωt), φ2 (ωt) of the roller assembly 46 change with time depending on the environment of use and the rotation phase angle of the inner joint member 44. In the case of transmitting a relatively great torque such as in a driving system of an automobile, the spin moments M1 and M2 cause the roller assembly 46 to incline by a larger angle. When the roller assembly 46 is largely inclined, the outer roller 48 comes in contact with the non-load side roller guide section 50 as shown in FIG. 10A, thus increasing the rolling resistance of the outer roller 48. This incurs an excessive frictional force inside the joint, which leads to an increase in tertiary rotational axial force. Such axial force often provokes vibration (so called “shudder”) of the vehicle in which the tripod type constant velocity universal joint 41 is incorporated.
A solution to avoid such a phenomenon is, according to the cited document and as shown in FIGS. 8 to 10, providing a flange portion 52 on both sides of the bottom portion 51 of the track groove in a circumferential direction, so as to sustain a facet of the outer roller 48 inclined by the spin moments M1, M2 with the flange portion 52, thus to suppress the inclination of the roller assembly 46 to keep the outer roller 48 from contacting the non-load side roller guide section 50 to a maximal extent.(Referring to JP-A No. 2002-147482 for example)
In the conventional tripod type constant velocity universal joint 41, if the spin moments M1, M2 continue to be generated even after the roller assembly 46 has been inclined so much that a facet of the outer roller 48 contacts the flange portion 52 (see FIG. 10B), a frictional force is generated between the outer roller 48 and the flange portion 52. Referring to FIG. 11, when a facet of the outer roller 48 is in one-point contact with the flange portion 52 via a point in a region opposing the flange portion 52 (for example, A1, A2, A3), the inclination of the roller assembly 46 with respect to the flange portion 52 is maintained at the same angle as the maximum value φ2max (Ref. FIG. 10A) of φ2 (ωt). However, if a phase of the center of moment lm is displaced after the outer roller 48 has contacted a facet of the flange portion 52 via the point B1, the inclination of the roller assembly 46 with respect to the flange portion 52 exceeds φ2max. A maximum inclination of the roller assembly 46 with respect to the flange portion 52 is the maximum value φ1max (Ref. FIG. 9) of φ1 (ωt) created by the spin moment M1, which is reached when the outer roller 48 has made two-point contact with both flange portions 52, i.e. the points B1 and B2. While the outer roller 48 is in one-point contact with a facet of the flange portion 52 the contact point remains at B1, however since a phase of the center of moment lm is gradually displaced, a distance L from the contact point B1 between the outer roller 48 and the flange portion 52 to the center of moment lm (hereinafter, “distance between contact point and center L”) gradually becomes shorter. A minimum value of the distance between contact point and center L is the distance between the point B1 and the center of the spin moment M1. When the distance between contact point and center L becomes shorter, the contact force of the outer roller 48 applied to the flange portion 52 becomes greater, and resultantly an excessive frictional force may be generated between the outer roller 48 and the flange portion 52.